Battery charging method and apparatus using initial charging step with gradually increasing charging current, quick charging step with large charging current and final charging step with decreasing charging current

ABSTRACT

A battery charging method for charging a battery such as a lead acid storage battery quickly, and a battery charging apparatus used in carrying out the battery charging method, includes an initial charging process between times 0 to t1, a quick charging process between times t1 to t2 and a final charging process between times t2 to t3, carried out step-wise and continuously. In the initial charging process, a charging current whose volume of electricity is increased gradually is applied. In the quick charging process, a charging current whose volume of electricity is larger than the initial charging process is applied intermittently to a battery while watching a voltage. In the final charging process, a charging current whose volume of electricity is reduced gradually is applied to the battery when the voltage has reached a charge-end voltage. Thereby, it is possible to charge in a very short time, and to penetrate the charging into electrodes without damaging the battery.

TECHNICAL FIELD

The present invention relates to a battery charging method for charginga battery such as a lead acid storage battery quickly, and a batterycharging apparatus used in carrying out said battery charging method.

TECHNICAL BACKGROUND

Conventionally, as a battery charging method, there are aconstant-voltage charging method which applies a constant voltage to abattery for charging, and a constant-current charging method whichapplies a constant current continuously to the battery for charging.Though there are both merits and demerits in either of these methods, itis not easy to realize shortening of charging time considerably by anyof existing methods.

In recent years, from a view point of environment protection, practicaluse of a so-called electromobile vehicle which uses the batteries as adriving source is strongly desired. For wide use of the electromobile,while it is necessary to shorten the battery charging time a great deal,the battery charging method which complies with this requestsufficiently is still not developed at present.

Incidentally, a capacity of a battery is defined by an ampere-hourcapacity (Ah) and an hour rate. For example, when the battery capacityof 10 Ah/10 hour rate is discharged at a current of 1 A, it can be usedcontinuously for 10 hours. When this battery is discharged at a currentof 4 A, while the continuous usable time is 2.5 hours by calculation, ittakes, as a matter of fact, approximately 1.5 hours to reach thecharge-end voltage. In the same way, when discharged at a current of 10A, while the continuous usable time is 1 hour by calculation, apractical continuous usable time is approximately 35 minutes.

Similarly, this applies to charging, too. For example, when a battery of10 Ah/10 hour rate capacity is charged at a constant current of 1 A for10 hours, it reaches fully charged state. When it is charged at aconstant current of 10 A, the chemical reaction in the battery proceedsfaster than the calculation, and as a matter of fact, it approaches tothe fully charged state by a half of the calculated time of one hour.

However, when the charging current is simply increased, it is liable tobe overcharged and there is possibility of damaging electrodes of thebattery. Moreover, there is a problem that only the surface of theelectrodes is charged and the charging is not penetrated into theelectrodes.

The present invention has been devised in view of the above-mentionedproblems. It is the object, therefore, to provide a battery chargingmethod and its apparatus which is able to charge in a very short itemand to penetrate the charging into the electrodes without damaging abattery.

DISCLOSURE OF THE INVENTION

A battery charging method according to the present invention ischaracterized in that, an initial charging process for applying acharging current whose volume of electricity is increased gradually tostart charging of a battery, a quick charging process for applying thecharging current having the larger volume of electricity than theinitial charging process intermittently to charge the battery whilewatching the battery terminal voltage, and a final charging process forapplying the charging current whose volume of electricity is reducedgradually to finish charging of the battery, when the battery terminalvoltage has reached a charge-end voltage, are carried out step-wise andcontinuously.

In a battery charging method claimed in claim 2, the battery defect isjudged by watching the charging current in the initial charging process.

In a battery charging method claimed in claim 3, in the quick chargingprocess, a continuous charging period during which the intermittentsupply of charging current is continued, and a charging suspensionperiod during which the intermittent supply of quick charging current issuspended, are repeated alternately till the battery terminal voltagereaches the charge-end voltage.

A battery charging apparatus-according to the present inventioncomprises, a control switch provided in a charging circuit, a chargingcurrent detector, a terminal voltage detector of a battery, and acontrol unit which applies a control signal responsive to the chargingsteps to the control switch, in response to detected results of thecharging current detector and the terminal voltage detector, to controlthe charging current step-wise, said control unit comprising: firstsignal generating means for generating a control signal for applying aninitial stage charging current whose volume of electricity is increasedgradually; second signal generating means for generating a controlsignal for applying a charging current having a volume of electricitylarger than the initial stage charging current intermittently; thirdsignal generating means for generating a control signal for applying afinal stage charging current whose quantity of electricity is reducedgradually; first judging means for comparing the detected value by thecharging current detector with a reference value to judge a batterydefect; second jugging means for comparing the detected value by theterminal voltage detector with the charge-end voltage to judge that theterminal voltage of the battery has reached the charge-end voltage;first switching means for shifting the operation from the first signalgenerating means to the second signal generating means in response tothe judging operation of the first judging means; and second switchingmeans for shifting the operation from the second signal generating meansto the third signal generating means in response to the judgingoperation of the second judging means.

In a battery charging apparatus claimed in claim 5, the control unitcomprises, a period setting means which sets the continuous chargingperiod during which the intermittent supply of charging current iscontinued, and the charging suspension period during which theintermittent supply of charging current is suspended, alternately tillthe battery terminal voltage reaches the charge-end voltage.

According to a battery charging method and a battery charging apparatusof the present invention, since a battery is charged by applying acharging current having a large volume of electricity intermittentlyafter applying the initial stage charging current, charging can befinished in a short time and the charging is penetrated into electrodeswithout damaging the electrodes of the battery.

Since a terminal voltage of the battery is watched during the quickcharging and the quick charging process is moved to the final chargingprocess when the battery terminal voltage has reached the charge-endvoltage, overcharging is prevented and the battery is not damaged.

In a battery charging method claimed in claim 2, since the batterydefect is judged by watching the charging current in the initialcharging process, a quick charging can be carried out only on anappropriate battery.

In a battery charging method and a battery charging apparatus claimed inclaim 3 and claim 5, singe a charging suspension period during which theintermittent supply of the charging suspension period, chargingpenetrates more into the electrodes and spreads throughout the battery,thereby keeping the balance of charging between the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing the principle of battery chargingmethod of the present invention,

FIG. 2 is an electrical circuit diagram showing an example of circuitconfiguration of a battery charging apparatus according to the presentinvention,

FIG. 3 is a block diagram showing a schematic configuration of a controlunit,

FIG. 4 is a block diagram showing a specific example of a drive controlunit,

FIGS. 5(1)-5(3) explanatory views showing the principle of generation ofa control signal in a control signal generating unit,

FIG. 6 is a flow chart showing control procedures by a operation controlunit, and

FIGS. 7(1)-7(5) are time charts showing the circuit operation of a drivecontrol unit.

PREFERRED FORM FOR THE EMBODIMENT OF THE INVENTION

FIG. 1 shows the principle of battery charging method according to oneembodiment of the present invention.

In the figure, the lapse of time is plotted along the abscissa. In thisbattery charging method, an initial charging period "a" between times"0" to "t1", a quick charging period "b" between times "t1" to "t2" anda final charging period "c" between times "t2" to "t3" are carried outstep-wise and continuously.

A battery terminal voltage and charging current are plotted along theordinate. In the figure, a character Q (V) shows variations of thebattery terminal voltage at charging, and a character Q (I) showsvariations of mean value (hereinafter referred to as "means currentvalue") per unit time of the charging current at charging.

The initial charging process a is the process, whereby the chargingcurrent whose volume of electricity is increased gradually is applied tostart charging of the battery. In this embodiment, the charging currentis applied intermittently, and soon after starting the charging, a pulsewidth t_(N) of the pulse-shaped charging current is set to asufficiently small value, and as the time elapses, the pulse width T_(N)relative to a pulse interval t_(F) is set larger in a step-wise mannerat every fixed time to increase the volume of electricity or the meancurrent value gradually.

In the initial charging process a, a battery defect is judged bywatching the charging current, thus when the mean current value at thetime point, when a fixed time (say 30 seconds) has elapsed, has reacheda predetermined reference value, it is judged that the battery is normaland the charging process moves to the quick charging process b, and whenthe reference value is not reached, it is judged that the battery isabnormal and extraordinary measures such as stopping the charging aretaken.

The following quick charging process b is the process, whereby thebattery is charged by applying the charging current whose volume ofelectricity is larger than that of the initial charging process aintermittently while watching the battery terminal voltage. In thisquick charging process b, the pulse width t_(F) is set large, therebythe charging current is set to the larger volume of electricity (meancurrent value) than the initial charging process a.

A value (pulse height) of the charging current is set to two to fourtimes of the battery capacity, that is, in the case of battery havingthe capacity of 10 Ah/10 hour rate, it is set to two to four times of 10A.

Furthermore, in this quick charging process b, a continuous chargingperiod T1 (say approximately 10 seconds) during which the intermittentsupply of the charging current is continued, and a charging suspensionperiod T2 (say approximately 2 seconds) during which the intermittentsupply of the charging current is suspended are set alternately till thebattery terminal voltage reaches a charge-end voltage V_(E). The batteryterminal voltage is measured during each of the charging suspensionperiods T2, preferably just before starting the continuous chargingperiod T1. In the figure, a character P designates measuring time pointsof the battery terminal voltage.

The final charging process c is the process, whereby charging of thebattery is finished by applying the charging current whose volume ofelectricity is reduced gradually, when the battery terminal voltagereaches the charge-end voltage V_(E). In this embodiment, the volume ofelectricity (mean current value) is reduced gradually by applying thecharging current intermittently, and setting the pulse width t_(N)relative to the pulse interval t_(F) of the charging current smaller ina step-wise manner at every fixed time as the time elapses.

FIG. 2 is an example of circuit configuration of a battery chargingapparatus used in carrying out the abovementioned battery chargingmethod.

In the battery charging apparatus shown in the figure, a chargingcircuit comprises a primary circuit 1 and a secondary circuit 2connected by a transformer 3. Power input terminals 5, 5 connected to asingle-phase A.C. power source 4 are provided in the primary circuit 1,and battery connecting terminals 7, 7 connected to a battery 6 areprovided in the secondary circuit 2.

In the primary circuit 1, a control switch 8 consisting of asemiconductor switch such as SSR is provided. The control switch 8operates to turn on and off by a control signal given from a controlunit 9, and the timing of on-off operation against the control signalinput is selected automatically by a zero-cross point of the A.C. input.

In the secondary circuit 2, a rectifier 10 which performs all-waverectification of a secondary output of the transformer 3, a chargingcurrent detector 11 for detecting the charging current flowing throughthe secondary circuit 2 and a terminal voltage detector 12 for detectingthe terminal voltage of the battery 6 are provided. Detected values bythe charging current detector 11 and the terminal voltage detector 12are taken into the control unit 9.

In the figure, numeral 13 designates a power circuit for supplying aD.C. voltage necessary for the operation of the control unit 9, andincluding a transformer 14, a rectifier 15 and a voltage converter 16.

FIG. 3 is a block diagram showing a schematic configuration of thecontrol unit 9.

In the control unit 9, an operation control unit 20, a clock generator21, a drive control unit 22, a display control unit 23 and a timer 24are included. The control switch 8 is connected to the drive controlunit 22 and an LED group 26 is connected to the display control unit 23,The LED group 26 consists of a plurality of LEDs, which are illuminatedand indicate operating states of the battery charging apparatus underthe control of display control unit 23,

The operation control unit 20 is constituted by a microcomputer, andincludes a CPU 27 which is the main body of control and operation, a ROM28 storing programs and a RAM 29 which reads and writes data. To the CPU27, the charging current detector 11 and the terminal voltage detector12 are connected via a bus 25, besides the drive control unit 22,display control unit 2S and timer 24.

FIG. 4 shows a specific circuit configuration of the drive control unit22, consisting of a control signal generating unit 30, a period settingunit 31, a gate circuit unit 32 and a fall-edge detecting circuit 33,

The control signal generating unit 30 is for generating a control signalwhich controls the on-off operation of the control switch 8, andincludes a saw-tooth wave generating circuit 34, a comparator 35, asetter 36 and an inversion circuit 37.

FIG. 5 shows the generating principle of the control signal in thecontrol signal generating unit 30.

FIG. 5 (1) is a saw-tooth wave [1] generated by the saw-tooth wavegenerating circuit 34. The comparator 35 compares the size of a voltagelevel of the saw-tooth wave [1] with a set point TH set in the setter36. When the voltage level of the saw-tooth wave [1] is above the setpoint TH, as shown in FIG. 5(2), an output level of the comparator 35becomes "1", and when below the set point TH, the output level becomes"0". Comparison output [2] of the comparator 35 is inverted by theinversion circuit 37, and as the inversion output, a control signal [3]shown in FIG. 5 (3) is obtained.

When the set point TH is set to a high level, a pulse width of thecontrol signal [3] becomes larger than a pulse interval. Thus, in thequick charging process b, the charging current having a large volume ofelectricity is obtained by setting such a high level set point TH togenerate the control signal [3].

When the set point TH is set to a low level, the pulse width of thecontrol signal [3] becomes smaller than the pulse interval. Thus, in theinitial charging process a and the final charging process c, thecharging current whose volume of electricity increases and decreasesgradually is obtained by setting such a low level set point TH variablyand in step-wise at every fixed time to generate the control signal [3].

Returning to FIG. 4, the period setting unit 31 is for setting a periodduring which the control signal [3] is given to the control switch 8,and includes a counter 39 to which a clock signal CK is given via an ANDcircuit 38, first and second comparators 40, 41 which compare the countvalue of the counter 39 with set points set in the setters 42, 43. Theclock signal CK is given to the AND circuit 38 from the clock generator21, and the AND circuit 38 outputs the clock signal CK to the counter 39when a starting signal is given from the CPU 27.

In the initial charging process a and the final charging process c, anyvalues which are sufficiently large are set in the setters 42, 43, andin the quick charging process b, a value corresponding to the continuouscharging time T1 is set in one setter 42, and a value corresponding to asum of the continuous charging time T1 and the charging suspension timeT2, or a one cycle length T3 is set in the other setter 43.

The first and second comparators 40, 41 are that, when the count valueof the counter 39 reaches the set points of respective setters 42, 43,the comparison values become "1", the comparison output of the secondcomparator 41 is given to the counter 39 as a reset signal via an ORcircuit 44.

The comparison output of the first comparator 40 is given to an ANDcircuit 46 together with the starting signal via the inversion circuit45. By the output of the AND circuit 46, the gate circuit unit 32 iscontrolled to open and close, and the control signal [3] is given to thecontrol switch 8 as the output signal when the gate circuit is inopen-state.

The fall-edge detecting circuit 33 detects the fall output of the ANDcircuit 45 and outputs an edge detecting signal to the CPU 27. When theCPU 27 receives the edge detecting signal, takes in the detected valueof the terminal voltage detector 12 at an adequate timing.

FIG. 6 shows control procedures of the operation control unit 20, andFIG. 7 shows time charts of the drive control unit 22.

In FIG. 6, Step 1 (shown as "STi" in the figure) to Step 9 show thecontrol procedures in the initial charging process, Step 10 to Step 13show the control procedures in the quick charging process, and Step 14to Step 19 show the control procedures in the final charging process. Inthe following, the operation of the battery charging apparatus isparticularly described according to FIG. 6 and FIG. 7.

First, in Step 1 (shown as "STi" in the figure) of FIG. 6, the CPU 27judges whether the power is switched on or not, when it is judged "YES",after setting an initial set point TH in the initial charging process inthe setter 36 of the control signal generating unit 30, and setting anysufficiently large values in the setters 42, 43 of the period settingunit 31, the CPU turns on the starting signal [4] (refer to FIG. 7 (1)),and starts the timer 24 (Step 2 to Step 4 ).

In FIG. 7, reference character TM1 indicates a start timing of theinitial charging process a, thereby the control signal generating unitS0 generates the control signal [3] (refer to FIG. 7 (4)) having thepulse width responsive to the initial set point TH. Though the counter39 of the period setting unit 31 starts counting operation, since itscounted value is smaller than the set points of the setters 42, 43. Acomparison output [5] (refer to FIG. 7 (2)) of the first comparator 41remains as "0", and an AND output [6] (refer to FIG. 7 (3)) of the ANDcircuit 46 is "1". Thus, the gate circuit unit 32 is maintained inopen-state, and in the initial charging process a, the control signal[3] passes through the gate circuit 32 as it is and is given to thecontrol switch 8 as an output signal [7] (refer to FIG. 7 (5)) .

In the timer 24, the duration time (say 30 seconds) of the initialcharging process a is set, and in the next Step 5, it is judged whetherthe time of the timer 24 is up, and in the following Step 6, it isjudged whether a predetermined time has elapsed.

When a predetermined time has elapsed, it is judged "YES" in Step 6 andthe CPU 27 proceeds to Step 7 to set the next set point TH in the setter36 of the control signal generating unit 30, and generates the controlsignal [3] having the pulse width responsive to the set point TH.

By renewing the set point TH at every elapse of predetermined time, andgenerating the control signal having the pulse width responsive to theset point TH until the time of the timer 24 is up, the charging currentwhose quantity of electricity is increased gradually is obtained.

When the time of the timer 24 is up, it is judged "YES" in Step 5 andthe CPU 27 proceeds to Step 8 to take in the detected value by thecharging current detector 11, and compares with a predeterminedreference value. When the detected value is above the reference value,though it is judged "YES" in Step 9 and the CPU 27 moves to the quickcharging process b below Step 10, when the detected value is below thereference value, the CPU 27 judges the battery defect and performsextraordinary procedures such as suspension of charging.

In the first Step 10 of the quick charging process by the CPU 27 sets apredetermined set point TH in the setter 86 of the control signalgenerating unit 30, sets a value corresponding to the continuouscharging time T1 in one setter 42 of the period setting unit 31, sets avalue corresponding to one cycle length T3 in the other setter 43, andresets the counter 89 of the period setting unit 31.

In FIG. 7, reference character TM2 indicates a start timing of the quickcharging process b, thereby the control signal generating unit 30generates the control signal [3] having the pulse width responsive tothe set point TH. Though the counter 39 of the period setting unit 31continues the counting operation, until the counted value reaches theset point of the setter 42 corresponding to the continuous chargingperiod, the comparison output [5] of the first comparator 40 is "0", andthe AND output [6] of the AND circuit 46 is "1" (refer to FIGS. 7 (2),(3)). Thus, the gate circuit unit 32 is set to open-state, and thecontrol signal [3] passes through the gate circuit unit 32 as it is andis given to the control switch 8 as the output signal [7] (refer to FIG.7 (5)).

When the count value of the counter 39 becomes larger than the set pointof the setter 42, the charging suspension period T2 is extended, thecomparison output [5] of the first comparator 40 becomes "1" and the ANDoutput [6] of the AND circuit 46 becomes "0" (refer to FIGS. 7 (2),(3)). Thereby, the gate circuit unit 32 is set to close-state, thecontrol signal [3] is interrupted by the gate circuit unit 32 and theoutput signal [7] is "0" (refer to FIGS. 7 (4), (5)).

When the AND output [6] of the AND circuit 46 falls from "1" to "0", itis detected by the fall-edge detecting circuit 33 and the edge detectingsignal is outputted to the CPU 27. Thereby, it is judged "YES" in Step11, and the CPU 27 takes in the detected value by the terminal voltagedetector 12 to compare with the charging end voltage V_(E) (Step 12).When the detected value has reached the charging end voltage V_(E),though it is judged "YES" in Step 13 and the CPU 27 moves to the finalcharging process c below Step 14, when the detected value is smallerthan the charge-end voltage V_(E), the CPU 27 returns to Step 11 tocontinue the quick charging process b and stands by for the next edgedetecting signal.

When the count value of the counter 89 becomes larger than the set pointof the setter 4S in the quick charging process b, the comparison outputof the second comparator 41 becomes "1", and by this comparison outputthe counter 39 is reset. As a result, the count value of the counter 39becomes smaller than the set point of the setter 42, the comparisonoutput of the comparator 40 is changed to "0" and the AND output [6] ofthe AND circuit 46 becomes "1" (refer to FIGS. 7 (2), (3)). Thereby, thegate circuit unit 32 is set to open-state, and the control signal [3]passes through the gate circuit unit 32 as it is and is given to thecontrol switch 8 as the output signal [7], thus returning to thecontinuous charging period T1 from the charging suspension period T2(refer to FIG. 7 (5)).

While such operations are repetitively executed, when the detected valueby the terminal voltage detector 12 reaches the charge-end voltageV_(E), it is judged "YES" in Step 13, and the CPU 27 moves to the finalcharging process c below Step 14.

In the first Step 14 of the final charging process c, after setting afirst set point TH in the final charging process c in the setter 36 ofthe control signal generating unit 30, and setting any sufficientlylarge value in the setters 42, 4S of the period setting unit 31, the CPU27 starts the timer 24 (steps 14, 15).

In FIG. 7, reference character TM3 indicates the start timing of thefinal charging process c, thereby the control signal generating unit 30generates the control signal [3] having the pulse width responsive tothe first set point TH (refer to FIG. 7 (4)). Though the counter 39 ofthe period setting unit 31 starts a counting operation, since the countvalue is smaller than the set points of the setters 42, 43 thecomparison output [5] (refer to FIG. 7 (2)) of the first comparator 41remains as "0", and the AND output [6] (refer to FIG. 7 (3)) of the ANDcircuit 46 is "1". Thus, the gate circuit unit 32 is maintained inopen-state, and in the final charging process c, the control signal [3]passes through the gate circuit unit 32 as it is and is given to thecontrol switch 8 as the output signal [7] (refer to FIG. 7 (5)).

A predetermined duration time of the final charging process c is set inthe timer 24, and in Step 16 it is judged whether the time of the timer24 is up, and in the following Step 17, it is judged whether apredetermined time has elapsed.

When the predetermined time has elapsed, it is judged "YES" in Step 17and the CPU 27 proceeds to Step 18 to set a next set point TH in thesetter $6 of the control signal generating unit 30, and generates thecontrol signal [3] having the pulse width responsive to the set pointTH.

By generating the control signal having the pulse width responsive tothe set point TH at every elapse of predetermined time as renewing theset point TH until the time of the timer 24 is up, the charging currentwhose volume of electricity is reduced gradually is obtained.

When the time of the timer 24 is up, it is judged "YES" in Step 16 andthe CPU 27 proceeds to Step 19 to turn off the start signal [4] tofinish charging.

In the above-mentioned embodiment, though the charging current isapplied intermittently in both the initial charging process a and thefinal charging process c, it is not limited thereto, and the chargingcurrent may be applied continuously.

What is claimed is:
 1. A battery charging method comprising the stepsof:an initial charging step of applying a first gradually increasingcharging current, to start charging of a battery, a monitoring step ofmonitoring the charging current in the initial charging step afterelapse of a predetermined charging time, to provide a detected value, acomparing step of comparing said detected value with a predeterminedvalue, a quick charging step of thereafter intermittently applying asecond charging current which is larger than the first charging current,while monitoring a terminal voltage of the battery, to charge thebattery to a charge-end voltage, when said detected value is greaterthan said predetermined value in said comparing step, said quickcharging step including the steps alternately repeating:a chargingperiod during which an intermittent supply of the second chargingcurrent is continued, and a charging suspension period during which theintermittent supply of the second charging current is suspended, untilthe terminal voltage of the battery reaches the charge-end voltage, anda final charging step of thereafter applying a gradually decreasingcharging current, to finish charging of the battery.
 2. A batterycharging apparatus comprising:a charging circuit for causing a chargingcurrent to flow to a battery; a control switch connected to saidcharging circuit for selectively passing and cutting off the chargingcurrent flowing through said charging circuit to the battery; a chargingcurrent detector connected to said charging circuit for detecting thecharging current flowing through said charging circuit; a terminalvoltage detector connected to the battery for detecting a terminalvoltage of the battery; and a control unit which controls current supplyto the charging circuit by supplying control signals to the controlswitch in response to detection by the charging current detector and theterminal voltage detector, said control unit including:first signalgenerating means for controlling said control switch to cause a firstgradually increasing charging current, to be supplied through thecharging circuit to the battery, to start charging of the battery, firstjudging means for comparing a value of said first charging currentdetected by the charging current detector with a reference value todetermine a defect of the battery, second signal generating means forcontrolling said control switch to intermittently cause a secondcharging current larger than the first charging current to be suppliedthrough the charging circuit to the battery when the first graduallyincreasing charging current is greater than said reference value afterelapse of a predetermined amount of time, to charge the battery to acharge-end voltage, period setting means connected with the controlswitch for alternately setting a charging period during which theintermittent supply of said second charging current is continued, and acharging suspension period during which the intermittent supply of saidsecond charging current is suspended, until the terminal voltage of thebattery reaches the charge-end voltage, and third signal generatingmeans for controlling said control switch to cause a third graduallydecreasing charging current, to be supplied through the charging circuitto the battery after said charge-end voltage has been reached by saidsecond charging current, to finish charging of the battery.
 3. A batterycharging apparatus according to claim 2, further comprising switchingmeans for controlling said first through third signal generating meansto shift the operation from the first signal generating means to thesecond signal generating means when the first charging current detectedby said charging current detector reaches a predetermined value, and forswitching the operation from the second signal generating means to thethird signal generating means when the detected voltage by said terminalvoltage detector reaches said charge-end voltage.
 4. A battery chargingapparatus according to claim 3, further comprising:second judging meansfor comparing a value of said terminal voltage detected by the terminalvoltage detector with said charging-end voltage to determine that theterminal voltage of the battery has reached the charge-end voltage, andsaid switching means controls said first through third signal generatingmeans to shift the operation from the first signal generating means tothe second signal generating means in response to said first judgingmeans, and switches the operation from the second signal generatingmeans to the third signal generating means in response to said secondjudging means.